I was chatting via IM with some friends today about setting yourself away when on IM to avoid people, or going invisible, or just not signing in at all if you have work to do or want to avoid some folks. And I realized that I left out one of the coolest comments from the June BayCHI meeting. I think it was Simeon Yates who said “SMS conversations never end.” I think that simple statement does a lot to call out some of the major differences between something like an IM or chat session and the use of SMS. It’s always on and available, and there’s no presence info. Once you start an SMS conversation with someone that’s it, they can keep it going if they want to no matter where you are and your willingness to receive.
I’m talking in generalities here, obviously you can call your operator and get them blocked or go to the persons house and steal their cell phone. But the affordances for the basic techniques of managing interaction aren’t built into the SMS interface. The totality of that control lies in knowing your phone number. It really helps to explain why there’s a much different set of evaluations to make when starting an IM conversation than there is with starting an SMS conversation. And why spam over SMS tends to evoke a much stronger reaction than SPIM.